FIG. 12 is a schematic side view of prior art apparatus 1100 with a gas purge for optical components 1102. Optical components 1102, in chamber 1104, are used in operations related semiconductor fabrication. Optical components 1102 are stacked in chamber 1104 in axial direction AD. Respective outer circumferences 1106 of components 1102 are fixed with respect to side wall 1108 of housing 1110. To maintain the functionality and operational life of the optical components, the optical components must be kept clear of contaminant residues. This is particularly important for system using ultra-violet light and extreme ultra-violet light. Higher levels of oxygen in chamber 1104 also leads to failure of adhesives used to secure optical components.
Contaminants are introduced into chamber 1104 by a number of sources. Adhesives, typically used to fasten components 1102 with respect to the side wall, are one source of contaminants. Off-gassing of components in the chamber is another source. For example, sulfur off-gasses from metals in the chamber. Contaminants also are introduced into the chamber, for example by leaks in housing 1110 to ambient atmosphere. Purge gas is introduced at one end of chamber 1104, for example by inlet 1112 in end E1 of the chamber, to remove contaminants in the chamber. The gas purge also is used to control temperature, humidity and other aspects of the environment within the chamber. The purge gas flows in flow path 1114 through the chamber picking up contaminants in the chamber and on the optical components. The purge gas with the contaminants is eventually exhausted through exhaust port 1116.
However, the flow of the purge gas across all of the optical components results in cross-contamination of the optical components, that is, contaminants accumulate in the purge gas as the gas flows from end E1 to end E2 of the chamber. The accumulated contaminants present an increasingly greater contamination risk for downstream optical components. For example, purge gas flowing over component 1102B includes contaminants picked up from component 1102B plus contaminants picked up by the purge gas from component 1102A. Thus, the increased concentration of contaminants in the purge gas increases the likelihood that the purge gas will leave a contaminant residue on component 1102B. Purge gas flowing over component 1102C includes contaminants picked up from component 1102C plus contaminants picked up by the purge gas from components 1102A/B. Now, the concentration of contaminants in the purge gas is even greater and the likelihood of leaving of the purge gas leaving a contaminant residue on component 1102C is further increased. The purge gas flowing across component 1102I has accumulated contaminants from all the upstream volumes and component 1102I has the greatest likelihood of being subject to cross-contamination from the other volumes.
Since exhaust port 1116 is typically located near the inspection optical components at end E2, the accumulation of sulfur in the purge gas can result in the introduction of sulfur to the reticle/wafer plane. Components in the reticle/wafer plane are extremely sensitive to sulfur contamination. For example, even very low concentrations of sulfur can result in terminal damage to copper wafers.
Further, the impact of a leak in chamber 1104 is exacerbated by the axial flow of the purge gas. For example, a leak at point P in the side wall can result in purge gas flowing out of the chamber at point P, reducing the flow of purge gas to optical components downstream of point P. This reduced flow lessens the ability of the purge gas to pick up contaminant from the downstream optical components, thus increasing the risk of contaminant residue on the downstream components. In addition, contaminants can be introduced to the chamber from the ambient atmosphere via the leak. These contaminants are added to the contaminants already present in the purge gas from the upstream contaminants and add to the cross-contamination problem.
The axial flow in chamber 1104 limits the pressure of the purge gas at individual components in the chamber. For example, the purge gas can be introduced to chamber 1104 at a pressure up to the maximum pressure capacity of the chamber. However, the pressure available at each of the optical components can be significantly less. As a result, the purge gas pressure/flow at downstream optical components is significantly reduced. Also, the axial flow does not enable deterministic control of the purge process for individual optical components. For example, it is not possible to modify or customize the flow pressures and patterns across individual optical components.